Raccoon poxvirus is an indigenous orthopoxvirus (family: Poxviridae) that infects North American raccoons. The virus is an enveloped, double stranded DNA virus (Family-Poxviridae; Subfamily-Chordapoxviridae; Genus-Orthopoxvirus), first isolated from the upper respiratory tract tissue of 2 of 92 apparently healthy raccoons during a survey of wildlife at Aberdeen Proving Grounds, Maryland; see, Herman, 1964, Bacteriol. Proc. 64.sup.th Ann. Meeting of the Am. Soc. Microbiol., p. 117.
For several years, recombinant raccoon poxviruses have been used as delivery systems for heterologous (i.e., foreign) nucleic acid molecules. See, for example, Esposito et al., 1988, Virology 165, 313-316; Esposito et al., 1992, In: Vaccines 92, Cold Spring Harbor Laboratory, pp. 321-329; U.S. Pat. No. 5,266,313, dated Nov. 30, 1993, by Esposito et al.; and U.S. Pat. No. 5,348,741, dated Sep. 20, 1994, by Esposito et al., each of which is incorporated herein by reference in its entirety. Heterologous nucleic acid molecules encoding antigens have been inserted into the thymidine kinase (TK) gene by homologous recombination; see, for example, Esposito et al., 1988, ibid. Such an insertion inactivates the TK gene, markedly reducing raccoon poxvirus virulence. Previous studies have studied the efficacy of oral or parentally delivered recombinant raccoon poxviruses in a wide range of animals including foxes, bobcats, rabbits, domestic cats, piglets, sheep and non-human primates; see, for example, Esposito et al., 1992, ibid.
Although recombinant raccoon poxviruses have been delivered using subcutaneous, intramuscular, and oral routes of administration, there are drawbacks to each of these administration routes. Intramuscular and subcutaneous forms of vaccine delivery have been correlated with a sporadic incidence of injection-site sarcomas. Furthermore, intramuscular or subcutaneous delivery of recombinant raccoon poxviruses leads to the production of high titers of antibodies against the raccoon poxvirus itself. Such antibody production interferes with the ability to use raccoon poxvirus as a delivery system for subsequent immunizations. In addition, booster administrations are typically required to obtain an efficacious immune response. Delivery of recombinant raccoon poxviruses by the oral route also has several disadvantages: This route of immunization requires administration of high titers of virus (typically more than 10.sup.8 plaque forming units (pfu)) as well as the need for several immunizations in order to obtain an effective antibody response.
There remains a need for a viral vector-based delivery system that can elicit an efficacious and safe immune response, and in particular a mucosal immune response. Such a delivery system preferably would be administered mucosally, would elicit an efficacious immune response against the heterologous antigen encoded by the recombinant virus without requiring a booster dose, and would elicit a sufficiently low response against viral antigens so as to allow for subsequent administration of one or more additional recombinant virus(es) leading to an efficacious immune response against the additional heterologous antigen(s) encoded therein.